Ultrasonic sensors are used in motor vehicles, for example as parking assistance, a so-called proximity measurement capability in a distance range of less than 30 cm being a decisive functional requirement for that purpose. They are generally made up of a housing and a transducer element situated therein. The housing is customarily molded or milled from a metallic material, for example aluminum. For purposes of corrosion protection and enameling, it is coated with a primer. An electromechanical transducer element (e.g. a piezoelectric element) is attached to the bottom of the housing, for example, glued and contacted. The housing is filled with a damping material. An injected silicone foam is one possibility for this.
For a number of reasons, these manufacturing steps are technically not trivial. In particular, the chemical processes of gluing and foaming require exact parameters and are difficult to implement in production. The same applies to contacting the transducer elements using bonds by means of thermocompression welding (TCW), for example.
Instead of filling the interior of the housing with foam, it is possible to achieve damping by placing foam pieces into the housing. The production step of filling with foam would thus be replaced by a step which is simple to manage manually. However, it is evident that as a result such sensors have less favorable attenuation characteristics than designs filled with foam, which adversely impacts the critical functional requirement for proximity measurement capability.
Measurements of the spectral components of the attenuation processes have shown that a significant component is due to resonances outside of the working frequency (48 kHz) of the ultrasonic sensor, components at 30 kHz and 70 kHz being significant in particular. These are due to tilting and crumpling movements in the housing wall. In order to dampen the housing wall vibrations, housing fillers are also used which are made of damping materials and/or additionally contain such materials.
The transducer elements are contacted, for example by attaching a terminal lead to the metalized transducer element on the upper side of the piezoelectric element, the underside of the transducer element being fastened to the bottom of the housing by a glue. The metallic housing or the metallic diaphragm constitutes the second terminal or the second electrode (cathode). The second terminal lead is then soldered to the conductive housing (made of aluminum for example), being suitably connected to it by spot-drilling of the housing wall or attached to a housing stud, which is considered to be disadvantageous with respect to the number of components and the production expense.
In another example, the contacting of the underside of the transducer is accomplished by so-called peripheral contacting. To that end, the piezoceramic disc is completely plated with silver and a D-shaped separating cut (see FIG. 2) is made in the silver plating. This results in two surfaces on the upper side which are contactable using stranded conductors, bonds or other conductors. A disadvantage in this connection is the non-homogeneous field/force distribution in the piezoceramic, since the upper surface, due to the D-cut, is only partially covered by the one (anode) contact (non-homogeneous plate capacitor). Another disadvantage is that if the underside of the transducer element is improperly glued to the bottom of the housing (as a function of the thickness of the glue and roughness of the base), the underside of the transducer element is not in contact with frame ground (GND or ground) due to the peripheral contacting and the sensor is undesirably enabled to operate as a capacitor, making it sensitive to electrical interference fields. In addition it is seen as disadvantageous that two contacts in the range of the useful oscillation are produced on the upper side of the transducer element as a result of the peripheral contacting. In addition to the influence by the contact within the meaning of a coupled oscillator, in practice the resonance frequency is reduced in proportion to the weight contribution of the contacting (for example soldering points). For this reason, thermocompression welding with low ground contribution is functionally advantageous in series production.
Japanese Patent Application No. 2002238095 A describes an ultrasonic sensor having a cover, the housing being smooth-walled on the one hand and being designed with steps on the other, which require increased production expense. The cover may be introduced into the housing offset in order to dampen specific oscillation modes, the setting of this position signifying increased expense. The cover is smooth-surfaced, it being designed to be thicker than the pot diaphragm but otherwise it has no countersinks or concavities specified in greater detail. The concept provides that the oscillating body “pot wall,” which is open on one side, is supported “hard” on the upper surface, thus producing a bending wave which is suspended bilaterally (Drawing 6 and 7). The cover is glued on.
The terminals are attached to the transducer element and the housing in a manner which is not described in greater detail.
German Patent Application No. DE 296 14 691 U1 describes an ultrasonic sensor in which a felt insert is held by a cover disc above a transducer element. The transducer element is contacted via a direct connection and via a contact pin inserted into a weighting ring. The structure is complex and includes the construction of a pretensioned Teflon film.
German Patent Application No. DE 197 54 891 C1 describes an ultrasonic transducer having two damping materials over a transducer element. The housing wall is thick compared to the diaphragm, the housing wall containing a contact pin.
German Patent Application No. DE 101 25 272 A1 describes an ultrasonic sensor and its manufacturing method. Its transducer element is covered by three different layers, a cover covering three housing parts. A conductive housing part constituting the diaphragm is provided with a caulked terminal. This publication refers solely to the production of the decoupling ring between the oscillating diaphragm and housing by spray coating with silicone.
An ultrasonic transducer described in German Patent Application No. DE 197 44 229 A1 has a housing having a bead and a decoupling ring sealing the housing. In one embodiment, the decoupling ring simultaneously forms an insert as a damping material within the housing. In another embodiment, it has a cylindrical through opening which is filled with a damping material. The through opening is not specified in greater detail with respect to its geometry and function. The decoupling ring is used simultaneously as a mounting for the terminals of the transducer element, a terminal of the housing being present.
German Patent Application No. DE 44 34 692 A1 describes an example of the attachment of a piezoceramic to a metal plate of an ultrasonic sensor using a conductive glue.